net/mlx5e: Support smaller RX/TX ring sizes

We un-intentionally limited the minimum rings size too much.

TX minimum ring size reduced from 128 to 64.
RX minimum ring size reduced from 128 to 2.

Signed-off-by: Achiad Shochat <achiad@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>